Gastric pH influences the appearance of double peaks in the plasma concentration-time profiles of cimetidine after oral administration in dogs

Comparison of Ussing Chamber and Caco-2 Model in Evaluation of Intestinal Absorption Mechanism of Compounds from Different BCS Classifications

BACKGROUND

Oral bioavailability (F), which is evaluated by permeability and solubility, is one of the key parameters in drug discovery. Currently, Caco-2 and Ussing chamber are both used in the study of intestinal permeability of drugs at different stages of drug development. However, comparative research between the Ussing chamber and Caco-2 for predicting the intestinal availability data (Fa×Fg) in humans has not been reported.

METHODS

In the present study, we evaluated the permeability of 22 drugs in rat intestines by Ussing chamber and compared them with the reported permeability data from Caco-2. In addition, the active transport of gabapentin was evaluated by Ussing Chamber.

RESULTS

Intestine segments were selected by corresponding absorption site for Ussing chamber analysis. BCS Class I and II compounds were more absorbed in the duodenum and jejunum, and Class III and IV compounds were more absorbed in the ileum. Papp values in the Caco-2 model were moderately correlated with human Fa×Fg (R2=0.722), and the Papp of the rat in the Ussing chamber revealed a better correlation with human Fa×Fg (R2=0.952). In addition, we also used the Ussing chamber to identify the transporter of gabapentin, and the results showed that the active absorption of gabapentin was related to LAT1.

CONCLUSION

Ussing chamber combined with rat intestinal tissue would be a significant tool for predicting the intestinal absorption and metabolism of compounds with diverse physiochemical characteristics.

Pharmacokinetic study of high-dose oral rifampicin in critically Ill patients with multidrug-resistant Acinetobacter baumannii infection

PURPOSE

Although rifampicin (RIF) is used as a synergistic agent for multidrug-resistant Acinetobacter baumannii (MDR-AB) infection, the optimal pharmacokinetic (PK) indices of this medication have not been studied in the intensive care unit (ICU) settings. This study aimed to evaluate the PK of high dose oral RIF following fasting versus fed conditions in terms of achieving the therapeutic goals in critically ill patients with MDR-AB infections.

METHODS

29 critically ill patients were included in this study. Under fasting and non-fasting conditions, RIF was given at 1200 mg once daily through a nasogastric tube. Blood samples were obtained at seven time points: exactly before administration of the drug, and at 1, 2, 4, 8, 12, and 24 h after RIF ingestion. To quantify RIF in serum samples, high-performance liquid chromatography (HPLC) was used. The MONOLIX Software and the Monte Carlo simulations were employed to estimate the PK parameters and describe the population PK model.

RESULTS

The mean area under the curve over the last 24-h (AUC0-24) value and accuracy (mean ± standard deviation) in the fasting and fed states were 220.24 ± 119.15 and 290.55 ± 276.20 μg × h/mL, respectively. There was no significant difference among AUCs following fasting and non-fasting conditions (P > 0.05). The probability of reaching the therapeutic goals at the minimum inhibitory concentration (MIC) of 4 mg/L, was only 1.6%.

CONCLUSION

In critically ill patients with MDR-AB infections, neither fasting nor non-fasting administrations of high-dose oral RIF achieve the therapeutic aims. More research is needed in larger populations and with measuring the amount of protein-unbound RIF levels.

Enhanced oral bioavailability of Bisdemethoxycurcumin-loaded self-microemulsifying drug delivery system: Formulation design, in vitro and in vivo evaluation

In this study, we sought to overcome the poor solubility and bioavailability of bismethoxycurcumin (BDMC) by fabricating a BDMC-loaded self micro-emulsifying system (BDMC-SMEDDS). Solubility and compatibility tests, pseudo-ternary phase diagrams (PTPDs) as well as d-optimal concept was applied to design the formulation. The assessment of the prepared BDMC-SMEDDS in-vitro mainly included droplet size (DS) and entrapment efficiency (EE) determination, morphology, drug release and stability testing. Besides, the in vivo behavior was also evaluated after oral administration of BDMC-SMEDDS to rats. The optimal formulation was found to compose of Kolliphor EL (K-EL, emulsifier, 645.3 mg), PEG 400 (co-emulsifier, 147.2 mg), ethyl oleate (EO, oil, 207.5 mg) and BDMC (50 mg). The BDMC-SMEDDS with satisfactory stability had a mean size of 21.25 ± 3.23 nm and EE of 98.31 ± 0.32%. Roughly 70% of BDMC was released from BDMC-SMEDDS within 84 h compared with <20% from the free BDMC. More importantly, the in-vivo behavior of BDMC-SMEDDS showed that the AUC_(0-12h) and plasma concentration of BDMC increased substantially as compared to the free BDMC. Altogether, BDMC-SMEDDS has the potential to enhance the solubility and bioavailability of BDMC and could be applied in the clinics.

Preparation, in vitro and in vivo evaluation of isoliquiritigenin-loaded TPGS modified proliposomes

Isoliquiritigenin (ISL) has a great variety of pharmacological effects especially liver cancer therapy, but its poor solubility, bioavailability and liver targeting have limited its clinical use. In order to solve the aforementioned shortcomings, the TPGS-modified proliposomes loaded with ISL (ISL-TPGS-PLP) was prepared in this study. ISL-TPGS-PLP was fabricated via thin-film dispersion method and was characterized by the appearance, particle size, zeta potential and morphology. HPLC was used to evaluate entrapment efficiency (EE), in vitro release and stability of ISL-TPGS-PLP single or combined while appropriate physicochemical parameters were measured with DLS. Meanwhile, the pharmacokinetics and tissue distribution were also studied after oral administration. The results demonstrated that ISL-TPGS-PLP had a mean size of 23.8 ± 0.9 nm, high EE of 97.33 ± 0.40%. More importantly, nearly 90% ISL was released from ISL-TPGS-PLP within 24 h while only 50% was released from ISL suspension. In the pharmacokinetics study, the area under the curve (AUC_0-24h) of ISL-TPGS-PLP was 1.53 times higher than that of ISL suspension. The Tissue distribution study showed that the ISL released from ISL-TPGS-PLP was higher in the liver than the free ISL suspension. Altogether, ISL-TPGS-PLP could ameliorate the ISL solubility, bioavailability and liver targeting ability, suggesting that ISL-TPGS-PLP could serve as a promising nanocarrier for liver cancer therapy.

Forecasting oral absorption across biopharmaceutics classification system classes with physiologically based pharmacokinetic models

OBJECTIVES

The aim of this study was (1) to determine how closely physiologically based pharmacokinetic (PBPK) models can predict oral bioavailability using a priori knowledge of drug-specific properties and (2) to examine the influence of the biopharmaceutics classification system class on the simulation success.

METHODS

Simcyp Simulator, GastroPlus^™ and GI-Sim were used. Compounds with published Biowaiver monographs (bisoprolol (BCS I), nifedipine (BCS II), cimetidine (BCS III), furosemide (BCS IV)) were selected to ensure availability of accurate and reproducible data for all required parameters. Simulation success was evaluated with the average fold error (AFE) and absolute average fold error (AAFE). Parameter sensitivity analysis (PSA) to selected parameters was performed.

KEY FINDINGS

Plasma concentration-time profiles after intravenous administration were forecast within an AAFE < 3. The addition of absorption processes resulted in more variability in the prediction of the plasma profiles, irrespective of biopharmaceutics classification system (BCS) class. The reliability of literature permeability data was identified as a key issue in the accuracy of predicting oral drug absorption.

CONCLUSION

For the four drugs studied, it appears that the forecasting accuracy of the PBPK models is related to the BCS class (BCS I > BCS II, BCS III > BCS IV). These results will need to be verified with additional drugs.

Using deconvolution to understand the mechanism for variable plasma concentration-time profiles after intramuscular injection

To introduce better antibiotics for the treatment of some infectious diseases in sheep and to expand the range of antibiotics available for veterinary medicine, pharmacokinetics of two antibiotics marbofloxacin (MBX) and trovafloxacin (TVX) were investigated in sheep after intramuscular injection. Variable and irregular plasma concentration-time profiles were observed for TVX but not for MBX. To understand the mechanism of this phenomenon, intravenous studies were performed for both drugs and data were analyzed using a population approach. Deconvolution was then performed using various approaches to obtain absorption profiles of both drugs in sheep after intramuscular injection. The Loo-Riegelman and staircase deconvolution function methods were found to provide more reliable estimates of absorption rate than the Spath-spline and B-spline constraining break points deconvolution methods. The absorption profiles resulting from deconvolution indicated a zero-order absorption process for TVX and a first-order process for MBX. Precipitation of TVX at the injection site was suspected to cause the pseudo zero-order absorption. This hypothesis was supported by the observation of crystalline deposits of TVX in sheep meat after direct injection, using reflectance confocal microscopy.

HPLC-MS/MS analysis of a traditional Chinese medical formulation of Bu-Yang-Huan-Wu-Tang and its pharmacokinetics after oral administration to rats

Bu-yang-huan-wu-tang (BYHWT) is one of the most popular formulated traditional Chinese medicine prescriptions, and is widely for prevention of ischemic cardio-cerebral vascular diseases and stroke-induced disability. A specific high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) has been developed and validated for simultaneous quantification of the nine main bioactive components, i.e., astragaloside I, astragaloside II, astragaloside IV, formononetin, ononin, calycosin, calycosin-7-O-β-d-glucoside, ligustilide and paeoniflorin in rat plasma after oral administration of BYHWT extract. This method was applied to investigate the pharmacokinetics in conscious and freely moving rats. No significant matrix effects were observed. The overall analytical procedure was rapid and reproducible, which makes it suitable for quantitative analysis of a large number of samples. Among them, three astragalosides and four isoflavones in A. membranaceus, ligustilide in Radix Angelicae Sinensis and Rhizoma Ligustici Chuanxiong and paeoniflorin in Radix Paeoniae Rubra were identified. This developed method was then successfully applied to pharmacokinetic studies of the nine bioactive constituents after oral administration of BYHWT extracts in rats. The pharmacokinetic data demonstrated that astragaloside I, astragaloside II, astragaloside IV and ligustilide presented the phenomenon of double peaks. The other herbal ingredients of formononetin, ononin, calycosin, calycosin-7-O-β-d-glucoside and paeoniflorin appeared together in a single and plateau absorption phase. These phenomenona suggest that these components may have multiple absorption sites, regulation of enterohepatic circulation or the gastric emptying rate, or there is ingredient-ingredient interaction. These pharmacokinetic results provide a constructive contribution to better understand the absorption mechanism of BYHWT and to support additional clinical evaluation.

Modelling the Double Peak Phenomenon in pharmacokinetics

Two methods of modelling the Double Peak Phenomenon in pharmacokinetics are described; both are based on compartmental models. The first method assumes that the absorption of the drug from the gut to the systemic plasma varies with the location of the drug in the gut, with negligible absorption through the jejunum. It has the advantage of clear physiological interpretation, but there are a comparatively large number of parameters to be estimated. The second method assumes simultaneous input via two parallel pathways, and has been developed with the aim of reducing the number of parameters in the model. However, this approach lacks the direct relationship to physiology. The two methods are used to model two data sets provided by AstraZeneca and a further data set from the literature, describing the pharmacokinetics of veralipride. For all three data sets, the measurement is of concentration of drug in the systemic plasma following oral administration in solution form.

Pharmacokinetics of cimetidine in dogs after oral administration of cimetidine tablets

Long-term oral treatment with cimetidine is recommended to reduce vomiting in dogs with chronic gastritis. Despite this, few studies have specifically examined the plasma disposition and pharmacokinetics of cimetidine in dogs, particularly following repeated oral administration. The pharmacokinetics of cimetidine following oral administration as tablets was investigated in healthy dogs. Cimetidine was absorbed rapidly post-treatment (t(max) = 0.5 h). A mean absolute bioavailability of 75% was calculated following a single oral administration of 5 mg cimetidine/kg body weight. After intravenous administration, a plasma half-life of 1.6 h was calculated. Repeated oral administration at the recommended dose rate and regime (5 mg/kg body weight three times daily) for 30 consecutive days did not lead to any accumulation of cimetidine in plasma. Food intake concomitant with oral administration of cimetidine delayed (t(max) = 2.25 h) and decreased the rate and extent of absorption (AUC) by about 40%. Cimetidine was well absorbed in fasted dogs. Administration of food decreased the bioavailability of cimetidine by 40%. Cimetidine does not accumulate over time in plasma when administered long term to dogs.

Whole-body physiologically based pharmacokinetic population modelling of oral drug administration: inter-individual variability of cimetidine absorption

Objectives

Inter-individual variability of gastrointestinal physiology and transit properties can greatly influence the pharmacokinetics of an orally administered drug in vivo. To predict the expected range of pharmacokinetic plasma concentrations after oral drug administration, a physiologically based pharmacokinetic population model for gastrointestinal transit and absorption was developed and evaluated.

Methods

Mean values and variability measures of model parameters affecting the rate and extent of cimetidine absorption, such as gastric emptying, intestinal transit times and effective surface area of the small intestine, were obtained from the literature. Various scenarios incorporating different extents of inter-individual physiological variability were simulated and the simulation results were compared with experimental human study data obtained after oral cimetidine administration of four different tablets with varying release kinetics.

Key findings

The inter-individual variability in effective surface area was the largest contributor to absorption variability. Based on in-vitro dissolution profiles, the mean plasma cimetidine concentration–time profiles as well as the inter-individual variability could be well described for three cimetidine formulations. In the case of the formulation with the slowest dissolution kinetic, model predictions on the basis of the in-vitro dissolution profile underestimated the plasma exposure.

Conclusions

The model facilitates predictions of the inter-individual pharmacokinetic variability after oral drug administration for immediate and extended-release formulations of cimetidine, given reasonable in-vitro dissolution kinetics.

Appearance of double peaks in plasma concentration-time profile after oral administration depends on gastric emptying profile and weight function

PURPOSE

Mechanism for double-peak occurrence in plasma concentration profile after oral administration of drugs is controversial, although irregular gastric emptying would be an important factor. The objective of this study was to assess the effect of gastric emptying and a weight function, i.e. pharmacokinetics after reaching the systemic circulation, on the double-peak appearance in plasma concentration profiles.

MATERIALS AND METHODS

Alprazolam, which generates irregular gastric emptying, was orally co-administered with theophylline to rats, and the plasma concentration profiles or absorption rates were compared between the two drugs. Both drugs are highly absorbable, but alprazolam is rapidly eliminated from plasma, while the elimination of theophylline is very slow.

RESULTS

Oral administration of alprazolam generated the irregular gastric emptying profiles, resulting in multiple peaks in the absorption rate profiles of both drugs. The double peaks in the absorption rate profiles led to the double peaks in plasma concentration profiles for alprazolam, but not necessarily for theophylline. Simulation study clearly indicated that the slower elimination from plasma made the first peak less recognizable.

CONCLUSIONS

The irregular gastric emptying could be a main reason for the double peaks in plasma concentration profiles. However, the frequency of double-peak occurrence depends on the weight function, particularly the elimination rate, of each drug.

Biowaiver Monographs for Immediate Release Solid Oral Dosage Forms: Cimetidine**This paper reflects the scientific opinion of the authors and not the policies of regulating agencies

Literature data relevant to the decision to allow a waiver of in vivo bioequivalence (BE) testing for the approval of immediate release (IR) solid oral dosage forms containing cimetidine are reviewed. According to the current Biopharmaceutics Classification System (BCS), cimetidine would be assigned to Class III. Cimetidine's therapeutic use and therapeutic index, its pharmacokinetic properties, data related to the possibility of excipient interactions, and reported BE/bioavailability (BA) problems were also taken into consideration. On the basis of the overall evidence, a biowaiver can be recommended for cimetidine IR products, provided that the test product contains only those excipients reported in this paper in their usual amounts, and that the test and the comparator drug products both are "rapidly dissolving" as per BCS.

Feasibility of Biowaiver Extension to Biopharmaceutics Classification System Class III Drug Products

BACKGROUND

The extension of biowaivers (drug product approval without a pharmacokinetic bioequivalence study) to drugs belonging to Class III of the Biopharmaceutics Classification System (BCS) is currently a subject of much discussion.

OBJECTIVES

To assess the relationship between in vitro dissolution characteristics and in vivo absorption performance of immediate-release (IR) products containing cimetidine, a BCS Class III compound, in human subjects. To evaluate the feasibility and appropriateness of an extension of the biowaiver concept to BCS Class III compounds.

STUDY DESIGN AND PARTICIPANTS

BCS-conform dissolution tests were carried out on ten marketed cimetidine products from Thailand and Germany, as well as cimetidine tablet formulations containing cimetidine 400mg manufactured by direct compression using methacrylate copolymer (Eudragit) RS PO) as a release-retarding agent to yield three batches with significantly different release profiles. Twelve healthy male subjects were enrolled in a randomised, open-label, single-dose schedule based on a five-way Williams' design balanced for carryover effects. Subjects received the following treatments, with 1-week washout periods between: (i) Tagamet 400mg tablet; (ii) 7.5% methacrylate copolymer cimetidine tablet; (iii) 15% methacrylate copolymer cimetidine tablet; (iv) 26% methacrylate copolymer cimetidine tablet; and (v) Tagamet (300 mg/ 2 mL) intravenous injection. The area under the plasma concentration-time curve from 0 to 12 hours (AUC(12)) and AUC from time zero to infinity (AUC(infinity)), peak plasma concentration (C(max)), absolute bioavailability (F) and mean residence time (MRT) were evaluated and statistically compared among formulations. In vitro-in vivo correlation (IVIVC) analysis was then applied to elucidate the overall absorption characteristics of each tablet formulation.

RESULTS

The release properties of the ten marketed cimetidine products were shown to comply with current US FDA criteria for rapidly dissolving drug products. As expected, the in vitro dissolution profiles of the cimetidine tablets containing different percentages of methacrylate copolymer differed considerably from one another. However, in vivo results showed no significant difference in AUC(12), AUC(infinity), C(max) and F between the tablets manufactured with methacrylate copolymer and the innovator. The MRT values obtained from 26% methacrylate copolymer tablets were significantly longer than for the other two methacrylate copolymer formulations and the Tagamet tablets. Furthermore, IVIVC analysis showed that the 26% methacrylate copolymer tablets exhibited dissolution rate-limited absorption, whereas the other formulations showed permeability rate-limited absorption.

CONCLUSION

The results of the present study indicated that the absorption of cimetidine from IR tablets is, in general, limited by permeability rather than dissolution. IVIVC analysis demonstrated that only when the release was deliberately retarded (tablets containing 26% methacrylate copolymer), did the dissolution represent the rate-limiting step to drug absorption. On the in vitro side, it seems that 85% dissolution within 30 minutes, as currently required by the US FDA Guidance, is more than sufficient to guarantee bioequivalence of IR cimetidine products. For cimetidine and other BCS Class III drugs with a similar intestinal absorption pattern, application of the biowaiver concept seems to present little risk of an inappropriate bioequivalence decision.

WAGNER-NELSON METHOD FOR ANALYSING THE ATYPICAL DOUBLE-PEAKED [13CO2] EXCRETION CURVE IN THE [13C]-OCTANOATE GASTRIC EMPTYING BREATH TEST IN HUMANS

1. In the [(13)C]-octanoic acid breath test, the time versus pulmonary [(13)CO(2)] excretion rate curve is analysed using mathematical curve-fitting techniques to calculate gastric emptying parameters. Thus, the goodness-of-fit highly influences the accuracy of the breath test. However, a double-peaked [(13)CO(2)] excretion curve, which occasionally develops owing to the presence of an interval of quiescent gastric emptying (the plateau phase), is likely to be fitted poorly. 2. In pharmacokinetics, the Wagner-Nelson method has been used to describe precisely the absorption kinetics of orally administered drugs and its reliability is independent of the nature of gastric emptying. A recent study has shown the potential of the Wagner-Nelson method to generate a realistic gastric emptying flow curve from [(13)CO(2)] excretion data. In the present report, we have demonstrated that the Wagner-Nelson method can visualize the plateau emptying phase responsible for the cases of double peaks. 3. Wagner-Nelson analysis applied to the breath test described precisely the characteristic emptying pattern of the two emptying phases being interrupted by the plateau phase. Conventional analysis for the breath test failed to detect the plateau phase. 4. The Wagner-Nelson method is a useful tool for analysing atypical double-peaking [(13)CO(2)] excretion curves.

Biopharmaceutics of β-cyclodextrin derivative-based formulations of acitretin in sprague-dawley rats

Acitretin, an active metabolite of etretinate, is as effective as etretinate in the treatment of psoriasis. Recently, we developed some water-soluble formulations of acitretin with 2-hydroxypropyl-beta-cyclodextrin (HPBCD)/randomly substituted methyl-beta-cyclodextrin (RMBCD). In this study, the biopharmaceutic properties of these formulations were tested in Sprague-Dawley rats. After single intravenous dosing (2.5, 5, or 10 mg/kg) with the HPBCD-based formulation, the area under the plasma concentration-time curve of acitretin increased proportionally with the dose and its clearance remained unchanged within the tested dose range. We also found that the RMBCD-based formulation of acitretin improved its bioavailability and decreased the variations in various pharmacokinetic parameters. The improved biopharmaceutic properties of RMBCD-based acitretin might be attributed to its enhanced aqueous solubility. The elimination of acitretin through bile excretion was also studied. Our results indicated that the major fraction of acitretin (approximately 40%) was excreted in the bile as beta-glucuronide conjugate and only trace amounts were excreted as unconjugated acitretin (approximately 0.5%). This finding further confirmed the importance of conjugated metabolism and biliary excretion in the elimination of this drug.

Influence of cimetidine co-administration on the pharmacokinetics of acebutolol enantiomers and its metabolite diacetolol in a rat model: the effect of gastric pH on double-peak phenomena

Acebutolol (AC) is a chiral beta-adrenergic receptor-blocking agent, which has been shown to be clinically effective in hypertension. The plasma concentration-time profiles of AC exhibit two peaks following oral administration of racemate for both R- and S-enantiomers. In the present study, the absorption of AC after a single dose was studied as a function of gastric pH in male Sprague-Dawley rats. Furthermore, the effect of cimetidine (CIM) on pharmacokinetic parameters of AC and its metabolite diacetolol (DC) was evaluated. CIM (50 mg kg(-1)) was administered via jugular vein 30 min prior to AC administration to elevate the intragastric pH. AC (50 mg kg(-1)) was administered orally by gavage and serial blood samples were collected before and for 8h after AC administration. Plasma samples were assayed for AC and DC, pharmacokinetic parameters were estimated and compared with those of control. The concentration-time profiles and the pharmacokinetics of AC were unchanged after co-administration of CIM. The oral absorption of AC, as assessed by the area under the plasma concentration-time curve (AUC) and the amount of unchanged drug recovered in the urine were not affected by CIM. The amount of metabolite recovered in the urine and the rate of absorption, however, were significantly altered. These are unlikely to be of clinically importance as we have found that the extent of absorption was not changed. We, therefore, concluded that intragastric elevation of pH has no effect either on generation of multiple peaking or on pharmacokinetic parameters of AC.

Evidence for dissolution rate-limited absorption of COL-3, a matrix metalloproteinase inhibitor, leading to the irregular absorption profile in rats after oral administration

PURPOSE

This study was undertaken to elucidate the underlying mechanism of the irregular absorption profiles of COL-3, a matrix metalloproteinase inhibitor, with a double- or plateau-peak concentration after a single oral dose administration of COL-3 suspension to rats.

METHODS

The gastrointestinal absorption profiles of COL-3 in rats were assessed by comparing serum drug concentration curves after the following various modes of drug administration: oral and intraduodenal doses, oral doses of COL-3 in fine and coarse suspensions, intraduodenal dosing to the bile-duct intact and cannulated (BDC) rats, and oral doses with and without food. In addition, the biliary excretion of COL-3 in the BDC rats was examined.

RESULTS

Neither variable gastric emptying nor enterohepatic recycling was the source of the irregular gastrointestinal absorption of COL-3 in rats. Reduction in particle size, presence of food and endogenous bile emerged as the determinants of the oral absorption of COL-3 by enhancing the dissolution of the solid drug in the gastrointestinal fluids. Flip-flop of the absorption and elimination rate constants was noted only for COL-3 after intraduodenal administration of the coarse suspension to the BDC rats with the bile flow diverged out of the body.

CONCLUSIONS

Variability in dissolution rate-limited absorption was the main cause of the irregular absorption of COL-3 after oral administration of its solid dosage form.

Variability in cimetidine absorption and plasma double peaks following oral administration in the fasted state in humans: correlation with antral gastric motility

The role of gastrointestinal motility and pH in determining cimetidine bioavailability as well as double peaks in plasma profiles following oral administration, in the quiescent or active phase of antral motility, to humans in the fasted state was examined. Plasma cimetidine-time curves did not show the presence of double peaks in any subject following intravenous administration. The incidence of double peaks was 73% following oral administration and was independent of antral migrating motility complex phase. Further, it was found that oral administration of cimetidine in the quiescent phase resulted in significantly higher bioavailability and in other pharmacokinetic parameters compared to that obtained following administration in the active phase. Excellent linearity in plots of motility peaks vs. plasma peaks with slopes close to unity were evident for both quiescent (r(2)=0.93) and active phase (r(2)=0.97) administration. A total of 14 peaks out of 22 (10 subjects, 64%) and 20 out of 27 peaks (11 subjects, 74%), were accounted for in quiescent and active phase oral administration, respectively. The proximal occurrence of plasma peaks to antral motility peaks typical of phase III contractions strongly implies that motility patterns may be responsible for secondary maxima following oral cimetidine administration in the fasted state.

Effect of elevated viscosity in the upper gastrointestinal tract on drug absorption in dogs

The objectives of these studies were, first, to determine the effect of elevated luminal viscosity on the gastrointestinal absorption of four model drugs and, second, to identify the key processes influencing drug absorption under elevated viscosity conditions. Studies were conducted in vitro and in healthy female mongrel dogs under fasting conditions. In the canine model, both the rate and extent of paracetamol and hydrochlorothiazide absorption were significantly decreased by the coadministration of 15 g guar gum dissolved in 500 ml normal saline. In the case of cimetidine, the rate but not extent of absorption was decreased. Owing to the high variability in the data, no statistically based conclusion could be drawn about the effects of coadministered guar gum on the oral absorption of the poorly soluble mefenamic acid. Based on the in vitro data, it appears that substantial reductions in the dissolution rate of paracetamol, hydrochlorothiazide and cimetidine account for the effects observed in vivo. It is concluded that the effect of an elevation in the intraluminal viscosity on drug absorption is greatest for highly soluble drugs, and results from a combination of a decrease in dissolution rate and gastric emptying rate.

Intestinal uptake of cimetidine and ranitidine in rats

The H2-receptor antagonists exhibit unusual absorption behavior in that double peaks often occur after oral administration. Moreover, administration with some high potency antacids decreases the extent of absorption. To date, no explanation that can completely account for these observations has been advanced. One problem is that there is a lack of consensus as to the mechanism of absorption of the H2-receptor antagonists from the gastrointestinal tract. In the studies reported here, the mechanism and regional dependence of intestinal uptake of two H2-receptor antagonists, cimetidine and ranitidine, were investigated in rats using the in vitro everted ring technique. The uptake rate of cimetidine from both jejunum and colon was linear with concentration (in the range of 0.0005-40 mM), and there was no significant competition for uptake in the presence of the structurally similar H2-receptor antagonists, famotidine and ranitidine. In the case of ranitidine too, the uptake rate from the jejunum and colon was linear with concentration (in the range of 0.0005-5 mM), and there was no competition for uptake by either famotidine or cimetidine. These data indicate that uptake of cimetidine and ranitidine in the rat jejunum and colon occurs by a predominantly passive process. Both cimetidine and ranitidine exhibited regional differences in uptake rate. Uptake tended to be greatest in the ileum, similar in duodenum and jejunum, and lowest in the colon. However, differences in uptake rates between locations in the small intestine appeared to be too modest to account for the double peak behavior of either compound.